1. Field of the Invention
The present invention relates to a motor driving device comprising a plurality of switching elements.
2. Description of the Related Art
A machine tool can process a workpiece while moving the workpiece or a tool. The machine tool comprises a spindle motor for rotating a spindle which holds the workpiece or the tool. Further, the machine tool comprises a feed axis motor for moving the workpiece or the tool in a direction of a predetermined feed axis.
The machine tool comprises a motor driving device for driving these motors. The motor driving device includes a semiconductor element, a capacitor, and the like for controlling an electric current supplied to the motor. The motor driving device is arranged, for example, inside a power magnetics cabinet in which an electromagnetic switch, a relay, a battery, or the like is arranged. As a semiconductor element for supplying and interrupting an electric current to the motor, a switching element is used. The switching element generates heat together with driving and thus is cooled by a heat sink, a fan, and the like.
In Japanese Laid-open Patent Publication No. 2014-165360, a servo amplifier is disclosed in which a heat sink connected to a single heat generation source is extended and connected to a housing. It is disclosed that the heat from the heat generation source is transferred to the housing, and thus the heat generation source is cooled in the servo amplifier.
In Japanese Laid-open Patent Publication No. 2001-345584, an electronic control device is disclosed in which a heat-absorbing heat sink is arranged on an upper side within a housing and a heat-dissipating heat sink is arranged on a lower side within the housing. It is disclosed that, in the electronic control device, the heat-absorbing heat sink and the heat-dissipating heat sink are connected by a heat conductive plate having excellent thermal conductivity.
In Japanese Laid-open Patent Publication No. 2003-243862, an electronic device is disclosed which comprises an internal heat sink to which a heating component is attached and which is arranged inside a housing and an external heat sink arranged outside the housing. The internal heat sink transfers the heat to the external heat sink via a heat conductive rubber sheet. It is disclosed that the heat generated by the heating component is released from the internal heat sink and the external heat sink in the electronic device.
When a plurality of motors are driven by one motor driving device, switching elements of which number corresponds to the number of motors are used. The switching element can be cooled by being mounted on a surface of a heat sink including a plurality of fins. It is preferable that the heat sink for cooling the switching element is arranged outside the housing of the motor driving device from the viewpoint of heat radiation.
However, when the number of feed axes of the machine tool is increased, and the number of the switching element is increased, all of the switching elements may not be mounted on one heat sink in some cases. Further, an area on which the heat sink is arranged is limited, and if many switching elements are cooled by the one heat sink, a cooling capacity per one switching element is lowered.
When all of the switching elements may not be mounted on the one heat sink, a plurality of heat sinks can be arranged. However, due to a restriction on a size of the motor driving device and the like, all of the plurality of heat sinks may not be provided outside the housing of the motor driving device in some cases. In such a case, the heat sinks are arranged inside the housing also.
The heat sink arranged inside the housing is often small due to a condition of a space to be arranged and low in a cooling capacity. In order to increase the cooling capacity of the heat sink arranged inside the housing, it can be considered to increase an air volume of a cooling fan. However, power consumption of the cooling fan is increased. In addition, generally, as the rotation frequency of the fan is increased, the life of the motor will be shorter, and reliability of the cooling fan is decreased. Therefore, when increase in temperature of the switching element becomes larger, output of the switching element needs to be suppressed.